Hi, for this question I don't understand how the mass of water is worked out. I have constructed an equation for it because I don't think this is necessary in a way... So at the end the product we have has 2 water molecule going from the starting product with 0.5 water molecule only. The answer is to simply multiply the n of plaster by 1.5. I don't quite get this, because this is not really the mole ratio though...Attachment 615024615022

It seems to me that you don't understand the concept of hydrated salts.

The "dot" between the formula and the water molecules means that the water (usually called water of crystallisation) is associated with the crystal structure in a ratio which is given by the coefficient (big number) in front of the water formula.

CuSO4.5H2O

This means that for every copper sulfate formula unit there are 5 associated water molecules. These molecules are used in building the crystal lattice (hence water of crystallisation)

Now if you multiply the whole formula up by Avogadro's number it then becomes 1 mole of hydrated copper(II) sulfate in which every 1 mole of copper(II) sulfate is associated with 5 moles of water molecules.

If you heat 1 mole of hydrated copper(II) sulfate strongly you will get 1 mole of copper(II) sulfate (anhydrous) and 5 moles of water given off as steam.

(Original post by charco)
I can't really add much to the previous comments.

It seems to me that you don't understand the concept of hydrated salts.

The "dot" between the formula and the water molecules means that the water (usually called water of crystallisation) is associated with the crystal structure in a ratio which is given by the coefficient (big number) in front of the water formula.

CuSO4.5H2O

This means that for every copper sulfate formula unit there are 5 associated water molecules. These molecules are used in building the crystal lattice (hence water of crystallisation)

Now if you multiply the whole formula up by Avogadro's number it then becomes 1 mole of hydrated copper(II) sulfate in which every 1 mole of copper(II) sulfate is associated with 5 moles of water molecules.

If you heat 1 mole of hydrated copper(II) sulfate strongly you will get 1 mole of copper(II) sulfate (anhydrous) and 5 moles of water given off as steam.

CuSO4.5H2O --> CuSO4 + 5H2O

Now apply these ideas to YOUR problem.

There are two forms of calcium sulfate:

CaSO4.0.5H2O
and
CaSO4.2H2O

What do you have to give to the first to get the second?

Thanks, I know what you mean now. So if there is 2CaSO4.0.5H2O, does it mean I have 1 mole of water in total? Thanks